The present invention relates to diamond and CBN (cubic boron nitride) compacts useful in manufacturing cutting tools and dies, and more particularly to improved diamond and CBN compacts of greater thickness and better integrity.
Diamond and CBN compacts for manufacturing cutting tools or wire drawing dies can be made by several techniques. One technique (British Pat. No. 1,382,080) places a mass of a metallic binder material in a high pressure apparatus upon which is placed a mass of particulate diamond. The masses are subjected to a pressure of greater than 10 Kbars at a temperature sufficient to melt the binder material for a time period of from 30 seconds to 3 minutes. The resulting diamond-metal compact is a composite of diamond crystals and a metallic binder which is dispersed mainly in the interstices between individual diamond crystals to bind such crystals together.
Another proposal for manufacturing such compacts (U.S. Pat. Nos. 3,745,623; 3,609,818; and 3,850,591) involves the sweep through catalyzed recrystallization method (STCR method). The STCR method disposes polycrystalline diamond or CBN preferably on carbide powder or a cemented carbide substrate disposed within a high pressure/high temperature apparatus. Alternately, the starting catalyst can be a single or multiple body or disc or annular shape in contact with the diamond or CBN powder. A small quantity of a bonding or catalyst metal selected from cobalt, nickel, and iron or aluminum alloy in the case of CBN is disposed in admixture with the abrasive crystals or with the carbide mass, or can be provided from the cemented carbide substrate. High temperature and high pressure in the diamond or CBN stable region then is applied for a time period generally exceeding 3 minutes. The resulting compact is characterized particularly by diamond-to-diamond or CBN-to-CBN bonding through the catalyzed recrystallization of the abrasive crystals being subjected to the process.
Methods for making cubic boron nitride compacts are disclosed in U.S. Pat. Nos. 3,743,489; 3,767,371; and 4,231,980.
For optimized strength of the compact product, very fine crystals of the abrasive typically are used, generally in particle size of less than 10 microns and preferably less than 5 microns. Such fine abrasive crystals crush extensively under the high pressures applied during the compaction process resulting in an initial packing density of around 1.5 grams/cc increasing to greater than 2.5 grams/cc by crystal fracturing. The resulting abrasive mass, therefore, is rather dense and offers resistance to the catalyst metal or catalyst metal and carbide from percolating or sweeping through the interstices therein. In practice, this sweep recalcitrance by the dense, fractured abrasive crystals leads to soft spots of non-bonded abrasive. These soft spots are especially prevalent when the layer of abrasive crystals exceeds about 1 mm in thickness. Coarser abrasive crystals offer channels in the compacted mass which are less torturous for the bonding metal to sweep through; however, surface finish and strength considerations usually preclude the use of such coarse crystals as starting materials for the compact. Addition of catalyst powder in making the compacts does not overcome the problem of soft spots or low final density.